1. Field of the Invention
This invention relates in general to fluid operated, percussion type drill apparatus and in particular to improved means for enabling the by-pass of fluid to cause cessation of piston-hammer reciprocation when the drill bit is suspended above the bottom of a borehole.
2. Description of the Prior Art
It has long been apparent that it is advantageous in percussion drilling to cease piston-hammer reciprocation and hammering on the associated anvil bit when the bit is suspended above the borehole bottom. This reduces the frequency of fatigue failure. In addition, it is advantageous to continue to exhaust fluid to the borehole bottom for removing cuttings that may otherwise accumulate when drilling ceases. These objectives have been accomplished in the prior art through the provision of various passages that enable fluid flowing through the motor to exhaust to atmosphere rather than drive the piston-hammer. One such arrangement is shown in U.S. Pat. No. 3,311,177. Another is shown in U.S. Pat. No. 3,595,323. Yet another is shown in U.S. Pat. No. 3,136,375. In these prior art devices, the exhausting passages are formed in the anvil bit itself, or in the motor housing or chuck means. Such arrangements require additional machining operations, add to manufacturing costs, and weaken the associated components.